In principle, the adaptive immune response can provide protective immunity to almost any non-self substance. In spite of this enormous potential, only a few dozen effective vaccines currently exist in the face of countless infectious agents, tumors, and disease processes that might be controlled by the induction of immunity to the pathogenic entity. Two factors are involved in determining a vaccine's efficacy: 1) the use of antigenic epitopes able to confer protective immunity, and 2) immunogenicity, or the capacity of the vaccine to induce an immune response to the antigens contained therein. In the last decade, it has become increasingly clear that the immunogenicity of an antigen depends largely on its presentation by dendritic cells.
Dendritic cells are not only the most potent antigen-presenting cells identified to date, but apparently the only ones that can activate naive (previously unstimulated) T cells in a primary immune response (Banchereau and Steinman 1998 Nature 392:245–252). Activation of naive T cells is necessary if a vaccine is to produce full T cell immunity and optimal antibody responses. Dendritic cells have this capacity due to their expression of high levels of the ligands required to activate naive T cells—namely, MHC:peptide complexes, co-stimulatory molecules and intercellular adhesion molecules (Sprent 1999 J Immunol. 163:4629–4636).
The problem for vaccine development is that dendritic cells are rare. They comprise approximately 1/400 cells in secondary lymphoid organs, 1/500 white blood cells and <1/1000 cells in most non-lymphoid organs. Their scarcity is compounded by the low frequency of naive T cells able to respond to any single antigenic epitope, or MHC:peptide complex, estimated to be 1/105 to 1/104 (Mason 1998 Immunol. Today 19:395–404). Hence, induction of an immune response depends upon antigen reaching one rare cell that must then interact with another rare cell, which would seem to militate against the development of immunity.
Naive T cells continuously recirculate through lymph nodes via the bloodstream (Gretz et al. 1996 J. Immunol. 157:495–499), whereas immature dendritic cells are relatively stable residents of non-lymphoid organs (Cowing and Gilmore 1992 J. Immunol. 148:1072–1079). Immature dendritic cells express low levels of surface MHC and co-stimulatory molecules and, as such, are only weak stimulators of T cell activation. However, these cells are actively pinocytic and phagocytic, enabling them to sample their environment for the presence of potential pathogens. When exposed to appropriate stimuli, immature dendritic cells are mobilized. Local tissue-specific adhesion molecules are down-regulated, permitting the cells to disengage from the tissue and migrate via afferent lymphatics to draining lymph nodes (Banchereau and Steinman 1998 Nature 392:245–252).
During their migration to lymph nodes, immature dendritic cells undergo “maturation” to become potent inducers of T cell activation. Maturation is characterized by 1) down-regulation of pinocytosis and phagocytosis, and 2) increased surface expression of MHC molecules that are loaded with peptides newly derived from proteins recently taken up from the environment. The expression of co-stimulatory molecules and intercellular adhesion molecules is up-regulated during maturation, while the pattern of chemokine receptor expression is altered, enabling the migrating dendritic cells to follow the correct route to the paracortical, T cell-rich areas of the draining lymph node (Banchereau and Steinman 1998 Nature 392:245–252). Once induced to migrate from their tissue of residence to the regional lymph node, mature antigen-bearing dendritic cells will be positioned to be encountered by antigen-specific naive T cells present in the recirculating pool of lymphocytes.
Langerhans cells are perhaps the best studied of the immature dendritic cells and serve as a prototype of immature dendritic cells in non-lymphoid organs. They reside in the epidermal layer of the skin and mucous membranes, where they are present in higher frequency (i.e., 1 to 2%) than the immature dendritic cells found in other non-lymphoid organs. Langerhans cells are bound to neighboring keratinocytes via the homophilic adhesion molecule E-cadherin (Udey 1997 Clin. Exp. Immunol. 107(Suppl. 1):6–8). This bond must be attenuated before the Langerhans cell can become mobile. Signals known to mobilize immature dendritic cells (e.g., IL-1, TNF-α, and LPS) have also been shown to decrease the expression of E-cadherin on Langerhans cell-like dendritic cells, inducing the loss of E-cadherin-mediated adhesion (Jakob and Udey 1998 J. Immunol. 160:4067–4073). Once released from surrounding keratinocytes, Langerhans cells pass through the basement membrane of the epidermis into the dermis, enter afferent dermal lymphatics and migrate to skin-draining lymph nodes. As detailed above, during this migration, Langerhans cells mature to acquire very high levels of surface MHC, co-stimulatory and adhesion molecules, and begin to express chemokines that attract naive T cells (Banchereau and Steinman 1998 Nature 392:245–252). Once in the draining lymph node, Langerhans cells remain there for a few days and then disappear (Ruedl et al. 2000 J. Immunol. 165:4910–4916).
Whether mature antigen-bearing dendritic cells will be encountered by and activate naive T cells in the lymph node is likely to depend on three factors: 1) the number of antigen-bearing dendritic cells that enter the node, 2) the density of MHC:peptide complexes expressed on their membranes, and 3) the frequency of antigen-specific T cells in the recirculating pool. Activation of naive T cells is a stochastic process, and the magnitude of the response increases with increasing density of MHC:peptide complexes on the antigen-presenting cell (Reay et al. 2000 J. Immunol. 164:5626–5634; Wherry et al. 1999 J. Immunol. 163:3735–3745). Similarly, the initial encounter between a dendritic cell and a naive antigen-specific T cell is most likely stochastic and should increase with increasing frequency of either cell type. For example, an administration of antigen that resulted in no detectable interaction between antigen-bearing dendritic cells and antigen-specific T cells in normal mice, was found to be immunogenic in mice that had an artificially high frequency of antigen-specific T cells (approximately 1/103) due to the transfer of T cells containing an antigen-specific T cell receptor transgene (Manickasingham and Reis e Sousa 2000 J. Immunol. 165:5027–5034). Based on the preceding considerations, a critical component of vaccine immunogenicity is the capture of vaccine antigens by rare, immature dendritic cells and the induction of their maturation and migration to draining lymph nodes, in numbers sufficient to be encountered by rare, antigen-specific T cells.
The induction of dendritic cell migration is a complex process that is incompletely understood at present, but certain signals have the capacity to mobilize or induce the migration of immature dendritic cells from their tissue of residence. They include the pro-inflammatory cytokines, TNF-α and IL-1, and bacterial lipopolysaccharide (LPS) (Kimber et al. 2000 Brit. J. Derm. 142:401–412). These signals, along with GM-CSF and other cytokines, initiate the maturation process as well. Physical trauma to a tissue, such as surgical excision, also may induce the migration and maturation of resident immature dendritic cells (Steinman et al. 1995 J. Invest. Dermatol. 105:2S–7S).
The paucity and functional immaturity of dendritic cells in non-lymphoid organs may explain why injection of an aqueous solution of most protein or peptide antigens results in little or no immunity and can even result in immunologic tolerance (Davila and Celis 2000 J. Immunol. 165:539–547; Garza et al. 2000 J. Exp. Med. 191:2021–2027; Liblau et al. 1997 Immunol. Today 18:599; Weiner 1997 Immunol. Today 18:335). Only a few dendritic cells are likely to be exposed to the antigen; and, in the absence of a stimulus for dendritic cell migration and maturation, those cells may never reach regional lymph nodes for recognition by recirculating T cells. Conversely, if the antigen is presented by cells that lack co-stimulatory and adhesion molecules, antigen-specific T cell tolerance can ensue. Genetic vaccines, comprising DNA or RNA encoding the antigen(s), also require processing of the protein product by host dendritic cells (Iwasaki et al. 1997. J. Immunol. 159:11) and thus are subject to the same constraints.
In summary, there is a need for an effective method to a) promote the capture of vaccine antigens by rare, immature dendritic cells, and b) induce the maturation of antigen-loaded dendritic cells and their migration to draining lymph nodes, in numbers sufficient to be encountered by rare, antigen-specific T cells. Such a method would function as an adjuvant to generate an adaptive immune response to an otherwise weak or non-immunogenic administration of antigen.